The fabric is very yellowed due to what looks like a later top coat of varnish or shellac. I write that as the sheen is uniform over both the doped fabric and the markings that would have been painted on top of the doped surface. Dope is applied to the linen fabric used to cover the wings and fuselage of the aircraft to tighten and stiffen the fabric. If the paint were to be applied first, the dope would not be able to penetrate the fabric, and those areas would flex differntly and not be as uniformly tight.